1. Field of the Invention (Technical Field)
The invention relates to a method and apparatus for determining when a storm has the potential for producing lightning, and more particularly to a method and apparatus for processing dual-polarization radar signals reflected from storms to detect the presence of electrically aligned particles that precede the occurrence of lightning within or from a storm.
2. Background Art
Most known systems for identifying electrified storms do so by detecting the occurrence of lightning within or from the storm. In these systems, lightning events are detected by sensing the transient electric or magnetic field changes which lightning produces, or by sensing higher-frequency electromagnetic radiation from the lightning at radio or optical frequencies. Examples of such systems are described in Ryan et al. (Paul A. Ryan and Nicholas Spitzer, "Stormscope," U.S. Pat. No. 4,023,408, issued May 17, 1977), Ryan et al. (Paul A. Ryan, Kenneth A. Ostrander and David L. West, "Storm Mapping System," U.S. Pat. No. 4,395,906, issued Aug. 2, 1983), Coleman (Ernest W. Coleman, "Storm Warning Method and Apparatus", U.S. Pat. 4,672,305, issued Jun. 9, 1987), and Krider et al. (E. Philip Krider, Alburt E. Pifer, and Leon G. Byerley III, "Thunderstorm Sensor and Method of Identifying and Locating Thunderstorms," U.S. Pat. No. 4,806,851, issued Feb. 21, 1989). Lightning data can be combined with conventional radar measurements of storm locations to identify which of several storms is producing lightning (George F. Andrews, "Spherics-to-Radar Data Converter," U.S. Pat. No. 3,508,259, issued Apr. 21, 1970). While the occurrence of lightning within or from a storm is a good indicator that the storm will produce additional lightning, it does not predict when the storm will be ready to produce the next lightning event. In addition, such methods and systems are unable to indicate when lightning will begin or end in a storm.
Alternatively, electrified storms can be identified by measuring the strong electrostatic fields which they produce. Lundquist et al. (Stig Adolf Svante Lundquist and Victor Scuka, "Thunderstorm Warning System," U.S. Pat. No. 3,611,365, issued Oct. 5, 1971) describes a system in which the electrostatic field from the storm is measured at a ground location and is used to trigger a warning, either from the electric field measurement itself when the field intensity exceeds a predetermined level or in combination with indications of lightning obtained in the manner described in the previous paragraph.
Thunderstorms are known to produce lightning as a result of the accumulation or segregation of electric charges at locations primarily inside the storm or on the surfaces of the storm cloud. The charges increase the electrical stress (i.e. the electrostatic field) in and around the storm until some natural or artificial event causes a lightning discharge to be triggered. The event or events which trigger natural lightning are not well understood, but such events require the presence of an electrostatic field whose magnitude or strength exceeds some threshold value within a volume of space, usually inside the storm. Methods or systems which sense the presence of strong electrostatic fields can be used to anticipate the occurrence or possible occurrence of lightning. Such methods or systems could be used to give warning of the initial onset of lightning within or from a storm, or to ascertain when a storm is finished producing lightning, or to anticipate the occurrence of individual lightning discharges within or from the storm. In addition, the systems could be used to give warning of the danger of the artificial triggering of lightning by objects such as aircraft or rockets flying through or near regions of strong electrostatic field.
Systems which measure the electrostatic field at or near the ground surface can provide warning of the impending occurrence of lightning, particularly at the beginning of a storm, but only if the measurements are made below the storm or at close range from it. The electrostatic field strength at the ground is known to decrease with the third power of the distance from the storm charges, so that a doubling of the distance decreases the field strength by a factor of eight. Smaller amounts of local electrical charge, which provide no lightning threat but which are closer to the measurement location can produce comparable or larger field strengths than the storm charges, and make the detection of impending lightning increasingly difficult as the storm progresses, even beneath the storm. Such local charges are known to be produced by weak electrical discharges, or coronae, from objects on the ground, and are known to mask the build-up of the storm charges to the next lightning discharge. Other local charges are known to reside on precipitation particles which fall toward the ground during and after the storm. Charged precipitation continues to fall for a period of time after the storm has ceased to present a lightning threat, and produces some of the highest electric field strengths measured at the ground from a storm. This unnecessarily prolongs the period of perceived lightning threat as inferred from ground electric field measurements.
The disadvantages of ground electric field measurements can be overcome to a certain extent by placing the instruments used to measure the electric field strength on an airborne platform, such as an airplane, balloon, or rocket, and flying them through or positioning them within or near storms to be monitored. This approach has substantial logistical disadvantages associated with the difficulty and/or expense of making airborne measurements, such as knowing where to make the measurements and being able to be at the necessary locations, and being able to adequately monitor storms both in space and in time. In addition, there are safety considerations associated with the bulky instrumentation and dangerous conditions in around storms. Finally, there are difficulties in interpreting observations that are not obtained directly from the strong-field region of a storm, and in separating out the temporal and spatial variability of measurements made from a moving platform.
An alternative approach to sensing electrified conditions in storms was suggested by B. Vonnegut in "Orientation of Ice Crystals in the Electric Field of a Thunderstorm", Weather, Vol. 20, pp. 310-312, (1965) who noted that ice crystals in storms should align themselves with the direction of the local electric field, and proposed that this effect should be detectable by viewing the upper parts of a storm cloud through polaroid filters. D. J. Mendez in "Optical Polarization Induced by Electric Fields of Thunderstorms," Journal of Geophysical Research, Vol. 74, No. 28, pp. 7032-7037 (1969) reported evidence that partly supported the detection of such an effect.
Hendry and McCormick (A. Hendry and G. C. McCormick, "Radar. Observations of the Alignment of Precipitation Particles by Electrostatic Fields in Thunderstorms," Journal of Geophysical Research, Volume 81, pp. 5353-5357, 1976) of the National Research Council in Canada reported radar observations which indicated that particles were being electrically aligned in the upper levels of electrified storms. The observations were made using a dual-channel circular polarization radar which operated at a frequency of 16.5 GHz, and were obtained by coherently correlating the co-polar (e.g. right-hand circular) and cross-polar (left-hand circular) returns from a storm. Large correlation coefficients, indicative of a high degree of common particle orientation or alignment, were observed in the upper parts of storms. The correlation values decreased suddenly at the time of lightning discharges, indicating that the particle alignment was caused by the electrostatic field of the storm. The altitude and nature of the correlated radar signals led them to conclude that the aligned particles were ice crystals, which depolarized the radar signal as it passed through regions of aligned particles, thereby giving rise to a correlated cross-polar return. This interpretation was confirmed and refined in two later papers (McCormick, G. C., and A. Hendry, "Radar Measurement of Precipitation-Related Depolarization in Thunderstorms", IEEE Transactions of Geoscience Electronics, Vol. GE-17, No. 4, Oct. 1976).
Hendry and McCormick noted that their observations had implications on the operation of earth-satellite communications links, and their results gave rise to a number of studies of such communications links by other investigators (e.g., Arnold and Cox, D. C., and H. W. Arnold, "Observations of Rapid Changes in the Orientation and Degree of Alignment of Ice Particles Along an Earth-Space Radio Propagation Path", Journal of Geophysical Research, Vol. 84(C8), pp. 5003-5010, (1979)). They did not discuss the possibility of using the technique for anticipating the occurrence of lightning or for identifying electrified storms, nor did they address this question in their investigations. One reason for this was that their radar system had only a limited capability for processing and displaying the observations in real time. Rather, their analyses were conducted primarily after the fact using a computer to analyze recorded signals.
Weinheimer and Few (A. J. Weinheimer and A. A. Few, "The Electric Field Alignment of Ice Particles in Thunderstorms," Journal of Geophysical Research, Volume 92, pp. 14833-14844, (1987)) concluded from a theoretical study that electric fields should have the ability to align ice crystals, indicating that ice crystals would be aligned in strong electric fields.